Captive-Portal Identification in DHCP / RA
draft-ekwk-capport-rfc7710bis-01

Network Working Group                                          W. Kumari
Internet-Draft                                                    Google
Updates: 7710 (if approved)                                     E. Kline
Intended status: Standards Track                                    Loon
Expires: July 19, 2019                                  January 15, 2019

               Captive-Portal Identification in DHCP / RA
                    draft-ekwk-capport-rfc7710bis-01

Abstract

   In many environments offering short-term or temporary Internet access
   (such as coffee shops), it is common to start new connections in a
   captive portal mode.  This highly restricts what the customer can do
   until the customer has authenticated.

   This document describes a DHCP option (and a Router Advertisement
   (RA) extension) to inform clients that they are behind some sort of
   captive-portal device, and that they will need to authenticate to get
   Internet access.  It is not a full solution to address all of the
   issues that clients may have with captive portals; it is designed to
   be used in larger solutions.  The method of authenticating to, and
   interacting with the captive portal is out of scope of this document.

   [ This document is being collaborated on in Github at:
   https://github.com/wkumari/draft-ekwk-capport-rfc7710bis.  The most
   recent version of the document, open issues, etc should all be
   available here.  The authors (gratefully) accept pull requests.  Text
   in square brackets will be removed before publication. ]

Status of This Memo

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   This Internet-Draft will expire on July 19, 2019.

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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Notation . . . . . . . . . . . . . . . . . .   3
   2.  The Captive-Portal Option . . . . . . . . . . . . . . . . . .   3
     2.1.  IPv4 DHCP Option  . . . . . . . . . . . . . . . . . . . .   4
     2.2.  IPv6 DHCP Option  . . . . . . . . . . . . . . . . . . . .   4
     2.3.  The Captive-Portal IPv6 RA Option . . . . . . . . . . . .   5
   3.  The Captive-Portal Link Relation Type . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  IETF params Registration  . . . . . . . . . . . . . . . .   6
       4.1.1.  Registry name: Captive Portal Unrestricted Identifier   6
       4.1.2.  Registry name: Captive Portal API Link Relation Type    6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   7
   Appendix A.  Changes / Author Notes.  . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   In many environments, users need to connect to a captive-portal
   device and agree to an Acceptable Use Policy (AUP) and / or provide
   billing information before they can access the Internet.  It is
   anticipated that the IETF will work on a more fully featured protocol
   at some point, to ease interaction with Captive Portals.  Regardless
   of how that protocol operates, it is expected that this document will
   provide needed functionality because the client will need to know
   when it is behind a captive portal and how to contact it.

   In order to present users with the payment or AUP pages, the captive-
   portal device has to intercept the user's connections and redirect
   the user to the captive portal, using methods that are very similar

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   to man-in-the-middle (MITM) attacks.  As increasing focus is placed
   on security, and end nodes adopt a more secure stance, these
   interception techniques will become less effective and/or more
   intrusive.

   This document describes a DHCP ([RFC2131]) option (Captive-Portal)
   and an IPv6 Router Advertisement (RA) ([RFC4861]) extension that
   informs clients that they are behind a captive-portal device and how
   to contact it.

1.1.  Requirements Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

2.  The Captive-Portal Option

   The Captive Portal DHCP / RA Option informs the client that it is
   behind a captive portal and provides the URI to access an
   authentication page.  This is primarily intended to improve the user
   experience by getting them to the captive portal faster; for the
   foreseeable future, captive portals will still need to implement the
   interception techniques to serve legacy clients, and clients will
   need to perform probing to detect captive portals.

   In order to support multiple "classes" of clients (e.g.  IPv4 only,
   IPv6 only with DHCPv6 ([RFC3315]), IPv6 only with RA) the captive
   portal can provide the URI via multiple methods (IPv4 DHCP, IPv6
   DHCP, IPv6 RA).  The captive portal operator should ensure that the
   URIs handed out are equivalent to reduce the chance of operational
   problems.  The maximum length of the URI that can be carried in IPv4
   DHCP is 255 bytes, so URIs longer than 255 bytes should not be used
   in IPv6 DHCP or IPv6 RA.

   In all variants of this option, the URI SHOULD be that of the captive
   portal API endpoint, conforming to the recommendations for such URIs
   [cite:API] (i.e. the URI SHOULD contain a DNS name and SHOULD
   reference a secure transport, e.g. https).  A captive portal MAY do
   content negotiation [citation?] and attempt to redirect clients
   querying without an explicit indication of support for the captive
   portal API content type (i.e. without application/capport+json listed
   explicitly anywhere within an Accepts header [citation]).  In so
   doing, the captive portal SHOULD redirect the client to the value
   associated with the "user-portal-url" API key.

   The URI SHOULD NOT contain an IP address literal.

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   The URI parameter is not null terminated.

   Networks with no captive portals MAY explicitly indicate this
   condition by using this option with the IANA-assigned URI for this
   purpose (see Section 4.1.1).  Clients observing the URI value
   "urn:ietf:params:capport-unrestricted" MAY forego time-consuming
   forms of captive portal detection.

2.1.  IPv4 DHCP Option

   The format of the IPv4 Captive-Portal DHCP option is shown below.

       Code   Len           Data
      +------+------+------+------+------+--   --+-----+
      | code | len  |  URI                  ...        |
      +------+------+------+------+------+--   --+-----+

   o  Code: The Captive-Portal DHCPv4 Option (160) (one octet)

   o  Len: The length, in octets of the URI.

   o  URI: The URI for the captive portal API endpoint to which the user
      should connect (encoded following the rules in [RFC3986]).

2.2.  IPv6 DHCP Option

   The format of the IPv6 Captive-Portal DHCP option is shown below.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          option-code          |          option-len           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                      URI (variable length)                    .
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  option-code: The Captive-Portal DHCPv6Option (103) (two octets)

   o  option-len: The length, in octets of the URI.

   o  URI: The URI for the captive portal API endpoint to which the user
      should connect (encoded following the rules in [RFC3986]).

   See [RFC7227], Section 5.7 for more examples of DHCP Options with
   URIs.

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2.3.  The Captive-Portal IPv6 RA Option

   This section describes the Captive-Portal Router Advertisement
   option.

    0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Type      |     Length    |              URI              .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               .
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               Figure 2: Captive-Portal RA Option Format

   Type  37

   Length  8-bit unsigned integer.  The length of the option (including
      the Type and Length fields) in units of 8 bytes.

   URI  The URI for the captive portal API endpoint to which the user
      should connect.  This MUST be padded with NULL (0x00) to make the
      total option length (including the Type and Length fields) a
      multiple of 8 bytes.

3.  The Captive-Portal Link Relation Type

   Some captive portal network deployments may be unable to change, or
   unwilling to risk changing, the network infrastructure necessary to
   use any of the above options.  In such deployments, when clear text
   HTTP intercept and redirection are used, a Link relation header
   ([RFC8288], Section 3.3) MAY be inserted to convey to a HTTP client
   (user agent) the associated Captive Portal API URI.

   HTTP user agents MUST ignore this link relation in any context other
   than when explicitly probing to detect the presence of a captive
   portal.  Failure to do so could allow an attacker to inject a Captive
   Portal API URI other than the correct URI for a given network or for
   networks where there is no captive portal present at all.

4.  IANA Considerations

   This document requests two new IETF URN protocol parameter
   ([RFC3553]) entries.

   Thanks IANA!

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4.1.  IETF params Registration

4.1.1.  Registry name: Captive Portal Unrestricted Identifier

   Registry name: Captive Portal Unrestricted Identifier

   URN: urn:ietf:params:capport-unrestricted

   Specification: RFC TBD (this document)

   Repository: RFC TBD (this document)

   Index value: Only one value is defined (see URN above).  No hierarchy
   is defined and therefore no sub-namespace registrations are possible.

4.1.2.  Registry name: Captive Portal API Link Relation Type

   Registry name: Captive Portal API Link Relation Type

   URN: urn:ietf:params:capport-api

   Specification: RFC TBD (this document)

   Repository: RFC TBD (this document)

   Index value: Only one value is defined (see URN above).  No hierarchy
   is defined and therefore no sub-namespace registrations are possible.

5.  Security Considerations

   An attacker with the ability to inject DHCP messages could include
   this option and so force users to contact an address of his choosing.
   As an attacker with this capability could simply list himself as the
   default gateway (and so intercept all the victim's traffic); this
   does not provide them with significantly more capabilities, but
   because this document removes the need for interception, the attacker
   may have an easier time performing the attack.  As the operating
   systems and application that make use of this information know that
   they are connecting to a captive-portal device (as opposed to
   intercepted connections) they can render the page in a sandboxed
   environment and take other precautions, such as clearly labeling the
   page as untrusted.  The means of sandboxing and user interface
   presenting this information is not covered in this document - by its
   nature it is implementation specific and best left to the application
   and user interface designers.

   Devices and systems that automatically connect to an open network
   could potentially be tracked using the techniques described in this

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   document (forcing the user to continually authenticate, or exposing
   their browser fingerprint).  However, similar tracking can already be
   performed with the standard captive portal mechanisms, so this
   technique does not give the attackers more capabilities.

   Captive portals are increasingly hijacking TLS connections to force
   browsers to talk to the portal.  Providing the portal's URI via a
   DHCP or RA option is a cleaner technique, and reduces user
   expectations of being hijacked - this may improve security by making
   users more reluctant to accept TLS hijacking, which can be performed
   from beyond the network associated with the captive portal.

   By simplifying the interaction with the captive portal systems, and
   doing away with the need for interception, we think that users will
   be less likely to disable useful security safeguards like DNSSEC
   validation, VPNs, etc.  In addition, because the system knows that it
   is behind a captive portal, it can know not to send cookies,
   credentials, etc.  By handing out a URI using which is protected with
   TLS, the captive portal operator can attempt to reassure the user
   that the captive portal is not malicious.

6.  Acknowledgements

   This document is a -bis of RFC7710.  Thanks to all of the original
   authors (Warren Kumari, Olafur Gudmundsson, Paul Ebersman, Steve
   Sheng), and original contributors.

   Also thanks to the CAPPORT WG for all of the discussion and
   improvements.

7.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997, <https://www.rfc-
              editor.org/info/rfc2119>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC2939]  Droms, R., "Procedures and IANA Guidelines for Definition
              of New DHCP Options and Message Types", BCP 43, RFC 2939,
              DOI 10.17487/RFC2939, September 2000, <https://www.rfc-
              editor.org/info/rfc2939>.

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   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <https://www.rfc-editor.org/info/rfc3315>.

   [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
              IETF URN Sub-namespace for Registered Protocol
              Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
              2003, <https://www.rfc-editor.org/info/rfc3553>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007, <https://www.rfc-
              editor.org/info/rfc4861>.

   [RFC7227]  Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
              S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
              BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
              <https://www.rfc-editor.org/info/rfc7227>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017, <https://www.rfc-
              editor.org/info/rfc8288>.

Appendix A.  Changes / Author Notes.

   [RFC Editor: Please remove this section before publication ]

   From initial to -00.

   o  Import of RFC7710.

Authors' Addresses

   Warren Kumari
   Google
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: warren@kumari.net

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   Erik Kline
   Loon
   1600 Amphitheatre Parkway
   Mountain View, CA  94043
   US

   Email: ek@google.com

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